Technical Approach

The GEM-3 has two sensing channels, a receiver (RX) coil at the center and a small reference coil between two transmitter (TX) coils. The RX coil produces the target signal, while the reference coil monitors the TX primary field. The unit time of operation, called “base period”, is commonly 1/30 second and designed to minimize powerline noise. In a 50 hertz (Hz)-power country, the base period is typically 1/25 second. Both coil outputs are digitized at an Analog-to-Digital Converter (ADC) rate of 96 kilohertz in a 24-bit resolution. The digitized time series has a length of 3,200 over a base period of 1/30 second (i.e., 96,000/30). In its simplest form, the GEM-3 may work in FD for one base period and in TD for another base period. In this way, the sensor will collect both the FD and TD data at a sampling rate of 15 Hz, which should be sufficient for most UXO surveys. In the FD mode, the RX and reference channel time series are subjected to sine and cosine convolutions to obtain the inphase and quadrature responses at each transmitted frequency. A complex ratio of the two responses is expressed in a pulse per minute (ppm) unit for each frequency, the raw data output of the sensor. The current operating software (Windows-based WinGEM) automatically converts the ppm data into apparent conductivity at each frequency and apparent susceptibility at the lowest frequency employed.